# Molecular Cluster‐Controlled Quasi‐Epitaxial CZTSSe/CdS Heterojunction Enables 12.3% Efficiency of Flexible Solar Cells

## TL;DR

A new method for making flexible solar cells using CZTSSe and CdS materials improves efficiency to 12.3% by reducing defects at the material interface.

## Contribution

A quasi-epitaxial CZTSSe/CdS heterojunction is constructed via controlled CdS molecular clusters during chemical bath deposition.

## Key findings

- Controlling CdS molecular clusters suppresses interface defects and increases open-circuit voltage to 503 mV.
- Flexible CZTSSe solar cells achieve 12.3% power conversion efficiency with high mechanical stability.
- Ultrathin buffer layers enhance short-wavelength transmittance and increase short-circuit current density by 15%.

## Abstract

Flexible Cu2ZnSn(S,Se)4 (CZTSSe) solar cells have garnered significant attention in photovoltaics. Interface defects in CZTSSe/CdS heterojunctions drive carrier recombination, leading to substantial open‐circuit voltage (VOC
) loss. Herein, a deposition strategy is proposed to achieve quasi‐epitaxial heterojunctions by controlling the CdS molecular clusters during the chemical bath deposition (CBD) process. The nucleation rate and size of clusters are regulated by manipulating the stirring process and a sealed NH3 atmosphere. Small‐sized CdS molecular clusters, densely adsorbed onto the CZTSSe surface at a controlled low rate, pair with dangling bonds to form the quasi‐epitaxial heterojunction structure. This effectively suppresses interface defects and mitigates tunnel‐enhanced recombination, resulting in an increased VOC
of 503 mV. The epitaxial growth of CdS thin films facilitates the formation of ultrathin buffer layers, thereby enhancing the short‐wavelength transmittance of the window layer, resulting in a 15% boost in short‐circuit current density (JSC
). Finally, the flexible CZTSSe solar cell achieves a power conversion efficiency (PCE) of 12.3% and demonstrates exceptional mechanical stability, retaining over 95% of its initial efficiency after thousands of bending cycles. The developed quasi‐epitaxial heterojunction strategy suppresses interfacial recombination, offering a promising route toward high‐efficiency flexible kesterite solar cells.

By controlling the synthesis of CdS molecular clusters, a quasi‐epitaxial heterojunction is directly constructed during chemical bath deposition, enabling flexible kesterite solar cells to achieve an efficiency of 12.3%.